simply in terms of the general atmospheric circulation 

 over the northeast Pacific Ocean. Monthly mean surface 

 pressure charts typically show two well-developed pres- 

 sure cells. A high pressure system over the ocean shifts 

 northward and increases in strength from winter to sum- 

 mer. The center of the cell moves in a northwestward 

 direction. This shift in the large-scale anticyclonic cir- 

 culation results in the observed winter to summer rever- 

 sal in the alongshore component of wind off the coasts of 

 Oregon and Washington. A low pressure system is 

 situated over the southwestern United States. This 

 semipermanent thermal low is fully developed over the 

 Central Valley in California during the summer. Cy- 

 clonic circulation associated with the low leads to 

 equatorward surface wind stress parallel to the coast. 

 The amplitude of the annual cycle is large. During win- 

 ter, both of these pressure cells weaken. The high pres- 

 sure system moves southward and the coasts of Oregon 

 and Washington come under the influence of the intense 

 low pressure system in the Gulf of Alaska. 



The primary mechanism controlling the location and 

 strength of the wind stress maximum and the associated 

 coastal upwelling is described by seasonal variations in 

 the gradient between the two pressure cells. During the 

 winter, this gradient is weak. Strong heating over the 

 continent during summer deepens the low and increases 

 the amplitude of the onshore -offshore pressure gradient. 

 As a result of the northward shift and strengthening of 

 the high, and deepening of the low, the region of max- 

 imum wind stress moves from the area south of Point 

 Conception to the vicinity of Cape Mendocino. These 

 variations are, of course, a function of differential ocean- 

 continent heating related to the annual cycle of solar 

 radiation. 



Physical Implications 



Climate of the adjacent coastal regions is influenced 

 by upwelling. During summer, the dome of high pres- 

 sure which develops over the North Pacific Ocean favors 

 large-scale subsidence and a strong temperature inver- 

 sion over the west coast of the United States. This sup- 

 presses deep cloud formation and greatly inhibits 

 precipitation. Unpublished distributions of cloud cover, 6 

 summarized from ship observations, show a large on- 

 shore-offshore gradient in the total cloud amount. 

 Minima occur at the coast. The effect of the large-scale 

 subsidence is noted in the true desert climate of Baja 

 California, and in the almost complete lack of rainfall 

 along the coasts of California, Oregon, and Washington 

 during the summer. Coastal upwelling primarily in- 

 fluences the local climate of the nearshore zone within 10 

 to 20 km of the coast and contributes to the formation of 

 low stratus clouds and fog typical of much of the coast 

 along California and Oregon. 



'Distributions of monthly mean cloud cover over the California Current 

 are on file at Pacific Environmental Group, National Marine Fisheries 

 Service, NOAA, Monterey, CA 93940. 



A secondary mechanism may account for local inten- 

 sification of surface wind stress and persistence of coastal 

 upwelling over periods ranging from several weeks to a 

 few months (Bakun 1974). In a simplified positive feed- 

 back model, wind stress parallel to the coast brings cold 

 water to the surface and cools the adjacent air. The 

 resulting temperature contrast between the continent 

 and the ocean increases the local pressure gradient. 

 Alongshore surface winds are increased and upwelling is 

 enhanced (Ramage 1971). This mechanism may be 

 slightly modified by the effect of atmospheric stability. 

 In the summertime coastal upwelling zone, the air-sea 

 temperature difference is usually positive. This stable 

 stratification decreases the magnitude of the surface 

 wind stress. The resulting negative feedback may par- 

 tially offset the increase in surface winds associated with 

 the described changes in the local pressure gradient. 



The existence of maximum wind stress some 200 to 300 

 km from the coast is an interesting feature. Of course, a 

 maximum in the onshore -offshore pressure gradient off- 

 shore may explain this phenomenon. Positive feedback 

 associated with wind-induced upwelling extending hun- 

 dreds of kilometers off the coast may act to intensify the 

 alongshore winds. However, this feature also suggests a 

 coastal boundary layer which acts to frictionally retard 

 the winds near the coast, leading to a positive wind stress 

 curl in the nearshore region. 



A characteristic feature of the wind stress curl dis- 

 tributions is the occurrence of a line of zero curl at some 

 distance from the coast. Observations show positive curl 

 inshore of this line and negative curl in the offshore 

 region. A theoretical analysis suggests that a poleward 

 undercurrent along an eastern boundary is favored by 

 positive wind stress curl along the coast and a poleward 

 decrease in surface heating (Pedlosky 1974a). The 

 monthly distributions of wind stress curl presented in 

 this study are generally consistent with an equatorward 

 Sverdrup flow offshore and a poleward Sverdrup flow 

 near the coast, except in the region from Punta Eugenia 

 to Punta Baja, where the wind stress curl is negative. The 

 general pattern of positive wind stress curl along the 

 coast and observations of the California Countercurrent 

 (Wooster and Jones 1970; Wickham 1975) are consistent 

 with Pedlosky's theory. 



The Sverdrup transport balance expressed in Equation 

 (6) may provide a simple and reasonable explanation for 

 the existence of the current-countercurrent system ob- 

 served along the west coast of the United States. Trans- 

 port calculations, based on the July wind stress curl data 

 for the line of 1-degree squares extending offshore of 

 Cape Blanco (lat. 43°N), show a net southward trans- 

 port of 3.5 X 10 12 gs _1 . Within 300 km of the coast, an in- 

 tegrated northward transport of 2.2 X 10 12 g s"" 1 is re- 

 quired. Negative wind stress curl between long. 127°W 

 and 133°W is associated with a southward vertically in- 

 tegrated mass transport of 5.7 X 10 12 g s" 1 . These values 

 underestimate, by a factor of two, the total volume trans- 

 port of 10 sv (1 sverdrup = 10 6 m 3 s _1 ) for the California 

 Current suggested by Sverdrup et al. (1942:724). 



23 



